1. Field of the Invention
The present invention relates to resonant tags used in the prevention of shoplifting, and the like. More particularly, the present invention relates to resonant tags capable of being attached to small-sized products because of their small size.
2. Description of the Background Art
A monitoring system composed of a combination of a tag, which resonates with a wave of a radio frequency, with transmitting and receiving antennas has heretofore been used in retail stores, libraries, etc. for the purpose of preventing shoplifting. The resonant tag has a structure that a coil and a plate are formed with an electroconductive metal foil on one side of an insulating film, another plate is formed on the other side thereof, and an LC circuit is constituted as a whole, and resonates with a wave of a specific radio frequency. If a product attached with this tag passes through a monitoring region without effecting checking, it resonates with the radio-frequency wave transmitted from the transmitting antenna, and the receiving antenna detects this resonance to give an alarm. As the resonant frequency, a frequency of 5 to 15 MHz is generally adopted for reasons of easy distinction from various noise frequencies.
The conventional resonant tags are in the form of a rectangle of 32 mm.times.35 mm in demensions even in the smallest and are considerably large, and so such a tag has been hard to be attached to small-sized cosmetics such as lipsticks, jewelry, and the like. The reason for it is that a circuit, which resonates with a wave of 5 to 15 MHz and has a sufficient gain and dimensions desired for the market, has been unable to be formed.
On the other hand, EPO 142380A2 discloses a resonant tag in which a circuit has been formed on each side. This tag has substantially the same patterns on both sides of a dielectric film, said patterns having been formed in a coil turned reversely to each other when viewed from the same direction and almost superimposed on each other. When the circuits are formed on both sides in such a manner, not only the number of the spiral coils is doubled, but also a capacitor is formed between the coiled portions on the front and back sides of the film, said coiled portions being superimposed on each other. Therefore, there is no need to form a separate capacitor portion. However, even in this tag, the dimensions thereof cannot be reduced smaller than a certain size. More specifically, the mere formation of the circuits on both sides cannot provide a smaller-sized resonant tag having sufficient resonance property.